1. Field of the Invention
The present invention relates to ground mapping radar systems and more particularly to high resolution ground mapping radar systems utilizing electronically scanned antennas.
2. Description of the Related Art
Radar systems employing electronically scanned phased array antennas can be used on aircraft to map ground planes. As an aircraft flies over a ground plane, radar signals are emitted and reflected back to the aircraft. The radar system interprets these emitted and reflected signals and produces a map of the ground plane. The ground plane map is typically generated along ground radial lines to facilitate scan conversion to a TV display format. For coherent doppler beam sharpening (DBS) and synthetic aperture radar (SAR) maps, mapping along ground radial lines of a ground plane is accomplished by applying range-dependent phase reference functions which follow the ground radial lines.
Unfortunately, map resolution or map range coverage can be poor with ground mapping radar systems utilizing an electronically scanned phased array antenna. A particular problem arises when electronically scanned phased array radar systems are used on dynamic aircraft such as modern military bombers and fighters. These types of aircraft have the ability to perform at large roll angles, however, map illumination is especially poor at such angles. To provide acceptable map illumination, an aircraft typically must limit aircraft motion during the time when radar mapping is performed. Aircraft must essentially be non-maneuvering when mapping. With newer aircraft and various mapping scenarios, ground mapping during high dynamics is very important. Even with a level-flying aircraft, map illumination may be unacceptable for certain applications.
The above problems are caused by a deficiency of prior art radar systems which does not appear to be recognized in the art. The deficiency is that isogain lines of an electronically scanned phased array antenna will not in general be in alignment with ground radial lines. Here, an isogain line is defined as the ground footprint of the peaks of an antenna's azimuth patterns at each range line. An isogain line could deviate significantly from ground radial lines, especially at large aircraft roll. Large angular deviation produces rapid antenna gain falloff as a function of map range. This reduces high resolution map range coverage. When map range coverage along a ground radial line falls below the desired extent, a technique to spread the beam wider in azimuth can be used, however, this lowers antenna gain and degrades signal-to-noise performance of the resultant map. Larger azimuth beam spoiling (i.e., widening the beam) will also aggravate the doppler ambiguity problem associated with the proper selection of Pulse Repetition Frequency (PRF). This doppler ambiguity problem is routinely encountered in high resolution radar mapping.
Similar deficiencies not previously recognized in the prior art exist for radar systems employing noncoherent high-resolution map modes such as monopulse ground mapping (MGM). In this mode, the concern is angular deviation between the isonull line, i.e., the ground footprint of the antenna monopulse null, and ground radial lines. Large angular deviation increases the size of the data stabilization memory. Under extremely large aircraft roll motion, the angular deviation may become so large as to prevent mapping completely at near ranges.